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On a broad scale, the 1980s and 1990s were characterized by unusual wetness with short periods of extensive droughts, the 1930s and 1950s were characterized by prolonged periods of extensive droughts with little wetness, and the first decade of the 2000s saw extensive drought and extensive wetness
(moderate to extreme drought graphic,
severe to extreme drought graphic).

Historical temperature, precipitation, and Palmer drought data from 1895 to present for climate divisions, states, and regions in the contiguous U.S. are available at the Climate Division: Temperature-Precipitation-Drought Data page. These filenames begin with "drd964x" and end with "txt".

Overview

July 2012 was another warmer- and drier-than-average month (warmest and 28th driest July on record, based on data back to 1895) when weather conditions are averaged across the country. A strong high pressure system (High, or upper-level ridge) kept its stranglehold over much of the U.S. for most of the month, resulting in persistent warm anomalies (weeks 1, 2, 3, 4) which dominated most of the country. Descending air ("subsidence") associated with the High inhibited precipitation in the central states, but a few cool fronts triggered areas of rain along the southern and eastern peripheries of the High, and monsoon showers brought rain to its western periphery (weeks 1, 2, 3, 4).
The dry weather, in combination with increased evaporation caused by the record heat, expanded drought conditions in the West, Great Plains, and Midwest, while below-normal rainfall expanded drought conditions in Hawaii.
Nationally, the moderate-to-exceptional (D1-D4) drought footprint increased to about 53 percent of the country while the percentage in the abnormally dry to exceptional drought category generally held steady at about 71 percent. About 19 percent of the country was in the worst drought categories (D3-D4, extreme to exceptional drought), more than double the percentage from last month.
The Palmer Drought Index, whose data base goes back 113 years, is relied upon for drought comparisons before 2000. The July 2012 Palmer value of 57 percent is the largest percentage since December 1956 when 58 percent of the contiguous U.S. was in moderate to extreme drought.

Historical Perspective

As noted in more detail in the Historical Analog section below, the droughts of the 1930s were largely a succession of expansions and contractions, characterized as "waves" of very intense drought spreading over huge expanses of the country, then retreating to a relatively small epicenter. The first huge drought expanded from the eastern U.S., but by the middle of the decade drought waves expanded from the Plains. These were the widely-known "Dust Bowl" days.
The 1950s were much more persistent in their coverage. The 1950s drought dominated the early to middle decade, expanding and retreating (but more slowly) with the epicenter in the Southern Plains, particularly Texas and New Mexico, and into adjacent areas of Mexico. Much of the South and Gulf Coast was seriously involved in this drought regime. At its peak, most of the country was involved in the drought. Most drought was eradicated during a very wet 1957.
The droughts of the 2000s were large, intense, regional events, and did not emanate from, or return to, the same regions in the way that the droughts of the 1930s and 1950s did. Compared to the "Drought Decades" of the 1930s and 1950s, the drought features were smaller in size and more migratory from one part of the country to another, but their intensity was strong. However, the western U.S. did see persistent drought during the first half of the decade.

Palmer Drought Index

The Palmer drought indices measure the balance between moisture demand (evapotranspiration driven by temperature) and moisture supply (precipitation). The Palmer Z Index depicts moisture conditions for the current month, while the Palmer Hydrological Drought Index (PHDI) and Palmer Drought Severity Index (PDSI) depict the current month's cumulative moisture conditions integrated over the last several months.

In some respects, the dryness of Julyintensified the long-term dryness which had become evident in June. As seen on the July 2012 Palmer Z Index map, low precipitation and record hottemperatures (with the accompanying increased evapotranspiration) led to short-term drought across much of the central part of the country this month. Wet conditions are evident on the Z Index map along the periphery of the dry area — in parts of the Great Lakes, Appalachians, Gulf of Mexico coast, Southwest, and Pacific Northwest. The Palmer Z Index map also indicates July dryness in parts of the East Coast. Compared with the June 2012 PHDI map, the July 2012 PHDI map indicates that drought conditions deteriorated from the central Rockies to Ohio Valley and Great Lakes, but improved along parts of the Gulf of Mexico coast and in the Southwest.
The July 2012 PHDI map also reflects the long-term nature of the drought conditions. The Z Index and PHDI maps in combination show that precipitation brought relief to parts of the Southwest and Deep South drought areas, and parts of New England dried out, but for much of the rest of the country — drier-than-normal weather persisted over the existing drought areas and wetter-than-normal weather continued over parts of the already-moist Pacific Northwest.

Palmer Drought Model Potential Evapotranspiration

Precipitation minus Potential Evapotranspiration for July 2012 calculated using the Palmer Model.

Did You Know?

Potential Evapotranspiration

The Palmer drought indices measure the balance between moisture demand and moisture supply. Drought results from an imbalance between these two components. Precipitation provides the water supply. Water demand is usually measured by evapotranspiration (the amount of water that would be evaporated and transpired by plants). There is a distinction made between potential evapotranspiration (PE) and actual evapotranspiration (AE). The Palmer model uses Thornthwaite's equations to estimate PE from temperature. PE is the demand or maximum amount of water that would be evapotranspired if enough water were available (from precipitation and soil moisture). AE is how much water actually is evapotranspired and is limited by the amount of water that is available. AE is always less than or equal to PE, so PE is used for the water demand component of the drought equation.

In the Palmer model, if the amount of precipitation (P) during the month is greater than PE for the month, then the leftover P soaks into the ground to recharge soil moisture, and any left over after that runs off as streamflow. If P is less than PE, then moisture has to be drawn out of the soil to meet the PE demand. Hotter temperatures result in greater PE which requires more P just to meet the greater demand. Climates where PE is always greater than P are termed arid climates. The American Southwest is a typical arid climate.

Standardized Precipitation Index

The Standardized Precipitation Index (SPI) measures moisture supply. The SPI maps here show the spatial extent of anomalously wet and dry areas at time scales ranging from 1 month to 24 months.

The 1-month SPI shows the domination of high pressure over the central part of the country, with very dry conditions surrounded by wet conditions along its periphery (as described earlier). Dryness in northern New England and the southeast corner of the country is also evident at the one month timescale. The persistence of the High over the central U.S. for the past several months is reflected by dry conditions from the Great Plains to Midwest at 2 to 6 months, with dryness also spread into parts of the West. At 6 and 9 months, two core areas of dryness begin to manifest, one centered over the Ohio Valley and the other stretching from the Central Plains to Intermountain Basin, with pockets of dryness in the East. At 12 months there are two areas of dryness, one in the Southeast and the other stretching from the West Coast to Midwest. The pattern at 24 months is largely wet conditions along the northern tier states and dryness in the southern states. The ring of wetness along the edges of the country, which surrounds the central core dry area, is evident from 1 to 3 months. Wetness is observed in the Pacific Northwest throughout most of the time scales, in New England at 2 and 3 months, and across the Northeast at 12 and 24 months.

Agricultural and Hydrological Indices and Impacts

USDA topsoil moisture short to very short

USGS monthly streamflow percentiles

Drought conditions were reflected in numerous agricultural, hydrological, and other meteorological indicators, both observed and modeled.

Regional Discussion

July 2012 was characterized by below-normal rainfall across much of the Hawaiian Islands. Abnormally dry to moderate drought conditions expanded into the northern islands this month, with moderate to extreme drought affecting about 54 percent of the state. Longer-term conditions continued drier than normal (last 2, 3, 4, 6, 12, 24, and 36 months, year-to-date, and water-year-to-date), especially for the southern islands.

In Alaska, July 2012 was generally drier than normal along a strip covering the central third of the state, wetter than normal along the west coast, and had a mixed pattern in the eastern sections. At short time scales (2, 3, 4, 6, and 7 months) relative dryness was significant in the north central sections, while at longer time scales a dry pattern at interior stations becomes evident (12, 24, and 36 months, and water-year-to-date). An area of abnormal dryness covered the northern areas on the USDM map.

Over a tenth of the U.S. was very dry (the driest ten percent of the historical record) during July 2012. The national rank for July 2012 was the 28th driest July in the 118-year record. On a statewide basis, July 2012 ranked in the top ten driest Julys for five states — one of which was Maine, but four of them (Nebraska [second driest], Iowa [third], Illinois [fourth], and Missouri [ninth]) were in the Central Plains to Midwest agricultural belt under the influence of an upper-level high pressure ridge. Kansas, which ranked twelfth driest, was also affected by the ridge. Ten other states ranked in the driest third of the historical record.

The dryness has been accompanied by abnormally warm temperatures at all time scales. July 2012 ranked as the warmest July in the 1895-2012 record for Virginia and second warmest for seven other states. In total, 32 states ranked in the top ten warmest category and an additional twelve ranked in the warmest third of the historical distribution. Colorado had its warmest June-July and May-July, 28 states had their warmest February-July, 33 had their warmest January-July, and 24 had their warmest August-July. For the last twelve months (August-July), only Washington state was not in the warmest third of the historical record or hotter.

Arkansas statewide Palmer Z Index, May-July, 1895-2012.

As noted earlier, excessive heat increases evapotranspiration and exacerbates drought. The combination of heat and dryness drove the Palmer Z Index to record or near-record levels for the following states:

Notably, Palmer Z Index value for May-July 2012 in Illinois was more severe than that for 1988, even though 1988 period was drier than that of 2012. The reason for this is May-July 1988 ranked only 19th warmest, thus illustrating the importance of temperature in exacerbating drought.

Even though the 2012 drought has reached a spatial extent that rivals the
maximum extent of the 1950s drought, every drought is different. Historical
analogs to the current drought can be determined by comparing the spatial
pattern and intensity of various climate indicators using statistical tools such
as the correlation
coefficient and mean
absolute difference. When applied to the temperature pattern for
July 2012, several Julys from the 1930s (1934,
1935,
1936)
are a close match in many regions. July
1936 matches July
2012 fairly well for precipitation, and July 1954 matches July 2012
reasonably well for PDSI
and PHDI
(although there are other months from other years that match July 2012 for all
of these climate indicators). When looking at the seasons from January-July
through June-July collectively, the 1950s are the closest match for the Palmer
indices (PDSI and
PHDI) in 2012, and the 1950s and 1988 closely match the 2012 Palmer Z Index,
but there is less consistency for temperature and precipitation.

Using the ten percent criteria, the 1930s droughts started around May 1929 and ended in February 1942. Some part of the country experienced drought during the 1929-1942 period. Like the 2000s drought, the spatial extent of the 1930s drought waxed and waned. Again, if ten percent is used as a criteria, then the 1930s drought had three episodes, roughly 1929 to early 1933, mid-1933 to early 1938, and late 1938 to early 1942. During this time, ten percent or more of the country experienced moderate to extreme drought for a total of 148 months.

For the 1950s droughts, the ten percent criteria indicates the nationally-significant drought started around August 1950 and ended in September 1957. Some part of the country experienced drought during the 1950-1957 period. Unlike the 1930s and 2000s droughts, the spatial extent of the 1950s drought essentially expanded and stayed at a relatively large area for most of the drought episode (especially from late 1953 to early 1957). Again, if ten percent is used as a criteria, then the 1950s drought had essentially one episode. During this time, ten percent or more of the country experienced moderate to extreme drought for a total of 86 months. But the average monthly area was 36 percent, compared to 32 percent for the 1930s drought and 29 percent for the 2000s drought.

Pacific Islands: According to reports from National Weather Service offices, the Pacific ENSO Applications Climate Center (PEAC), and partners, conditions varied across the Pacific Islands.

On other Pacific Islands (maps — Micronesia, Marshall Islands, basinwide), July was drier than normal for Majuro, but near to above normal for the rest of the stations. Majuro has been drier than normal for the last three months. Total rainfall for the last 12 months (August 2011-July 2012) was near to above normal for all stations.